Various physical, chemical and mechanical methods have been devised for the synthesis of nanostructured powders (n-powders). These have been described in detail in the scientific literature (see "NanoStructured Materials," Vols. I, II and III, 1992-4). Of particular relevance to this invention is the prior art on the synthesis of n-powders by (1) thermal decomposition of metalorganic precursors using a focused laser beam, combustion flame or plasma torch as heat source, and (2) evaporation and condensation of volatile species in a reduced-pressure environment.
In the laser synthesis method, n-ceramic powders are produced by passing a precursor/carrier gas stream through a plasma generated by the interaction of a high power density focused laser beam with the gas stream. During the very short residence time of the precursor compound in the plasma, ultrafine clusters or nanoparticles are formed which are carried away by the exhaust gases to be collected in a filter system. Typically, the powders are monodispersed (mean particle size&lt;20 nm), loosely agglomerated, and readily sinterable to theoretical density by conventional hot pressing methods. The resulting sintered materials display excellent properties because of their high purity and ultrafine structures. The process is capable of producing a variety of n-ceramic powders on the laboratory scale, but does not appear to be suitable for the industrial-scale production of such powders.
In the combustion flame and plasma torch synthesis methods, n-ceramic powders are produced by direct injection of the precursor/carrier gas stream into the flame or plasma under ambient pressure conditions. An important difference in the two processes is the use of an oxidizing or reducing atmosphere in the flame process, and an inert or reactive gas atmosphere in the plasma process. The flame process has been applied successfully to the production of commercial quantities of carbon black, TiO.sub.2 and SiO.sub.2. The plasma process has been used to produce experimental quantities of non-oxide ceramics, including metal carbides, nitrides and borides, such as TiC, TiN and TiB.sub.2, as well as their refractory metal equivalents. The simplicity of these two processes more than compensates for the relatively high cost of the precursor chemicals, such as TiCl.sub.4 for the production of TiO.sub.2 by the flame process, or TiC and TiB.sub.2 by the plasma process. For the case of carbon black synthesis, this is not an issue because the carbon source gas is CH.sub.4 or natural gas.
A feature of both synthesis methods is the highly agglomerated state of the as-synthesized n-ceramic powders. For example, in the flame synthesis of TiO.sub.2, the primary powder particles can vary in size from 5-100 nm, but form cemented aggregates with from 10-1000 nanoparticles per aggregate. While for many applications the agglomeration of the n-ceramic powders is of little consequence, there are situations where it is a shortcoming. An example is in the fabrication of monolithic ceramic shapes for structural applications, where the presence of residual porosity in the sintered material seriously limits performance. It is known that the porosity is due to the bridging of agglomerates in the powder compacts prior to sintering.
In 1985, a potential solution to the nanoparticle agglomeration problem came with the introduction of the Inert Gas Condensation (IGC) synthesis method. In this process, an evaporative source is used to generate the nanophase particles and these are convectively transported to and collected on a cold substrate. An essential feature of the process is a reduced-pressure environment of inert gas, such as He, which must be maintained in the optimal range of 1-10 mbar for the efficient production of non-agglomerated n-particles. The particles are formed in a region just above the evaporative source by interactions between the hot evaporative species and the cold inert He gas atoms in the reaction chamber. Experimental quantities of high purity non-agglomerated n-powders of many different materials have been synthesized by the IGC process, and in several cases full densification of powders has been achieved by solid state sintering of powder compacts at temperatures as low as 0.5 Tm.
The challenge of scaling-up the process is now being addressed, using a forced convective flow system for continuous processing of powders, and a multi-kilowatt electron beam for achieving high evaporation rates. However, many useful ceramics and low vapor pressure metals cannot easily be produced by such an evaporation method, so some other approach is needed.